Using peroxidase-antiperoxidase (PAP) procedure designed to demonstrate the GFA protein by specific antibody the basic astrocytic reaction consisting in proliferation of astroglial fibrils was studied in cats subjected to the occlusion of the middle cerebral artery (MCA) or to cerebral embolism by injecting solid microspheres into the internal carotid artery. These studies revealed that the GFAP reaction appearing as a striking proliferation of fibrils specifically confined to astrocytes was demonstrable in cats sacrificed only 10 minutes after embolization. In cats with MCA occlusion and sacrificed after 6 hours extensive GFAP reaction was conspicuous in the regions of the brain affected by cerebral ischemia with the exception of the areas which showed extravasation of serum proteins.